With the popularization of information communication services, the emergence of various multimedia services, and the provision of high-quality services, demand for a wireless communication service has rapidly increased. To actively cope with such demand, the capacity of a communication system should be increased and the reliability of data transmission should be improved.
To increase communication capacity in wireless communication environments, a method for newly searching available frequency bands and a method for increasing efficiency for limited resources may be considered. As to the latter method, a multiple-input multiple-output (MIMO) scheme has recently drawn attention and has been actively developed. The MIMO scheme obtains a diversity gain by equipping a transmitter and a receiver with a plurality of antennas to additionally ensure a spatial region for utilizing resources, or increases transmission capacity by transmitting data in parallel via the plurality of antennas.
Generally, the MIMO scheme is considered to raise the reliability of a communication system or to improve transmission efficiency and may be classified into beamforming, spatial diversity, and spatial multiplexing schemes.
The beamforming scheme and spatial diversity scheme, which use multiple transmit antennas to raise reliability, transmit a single data stream through multiple transmit antennas. The spatial multiplexing scheme, used to raise transmission efficiency, simultaneously transmits multiple data streams via multiple transmit antennas.
In the spatial multiplexing scheme, the number of simultaneously transmitted data streams is called a spatial multiplexing rate. The spatial multiplexing rate should be appropriately selected according to the number of transmit and receive antennas and to a channel state. Generally, the spatial multiplexing rate which can maximally be obtained is limited to a smaller value of the number of transmit antennas and the number of receive antennas. If correlation of a channel is increased, a low spatial multiplexing rate is used.
When employing the spatial multiplexing scheme, various gains can be obtained by applying a virtual antenna signaling scheme. For example, since channel environment of multiple data streams become the same by application of the virtual antenna signaling scheme, robust channel quality information (CQI) can be provided and the reliability of a data stream having a bad channel state can be increased.
Further, a transmit power of physical antennas to which a virtual antenna signaling scheme is applied can be nearly uniformly maintained. In more detail, sets of physical transmit antennas form a plurality of beams each corresponding to a virtual antenna. Different beams are generated not only to transmit the same power from all the physical antennas but also to reserve a channel characteristic.
The total number of virtual antennas determines an available spatial diversity or spatial multiplexing rate. Moreover, the total number of virtual antennas determines the amount of overhead required to measure space channels. Hereinbelow, the number of physical transmit antennas is denoted by Mr, the number of available virtual transmit antennas is denoted by Me, and the number of simultaneously transmitted layers is denoted by M. The layer indicates a transmission symbol which is independently coded and modulated for transmission.
Meanwhile, a precoding scheme refers to a spatial processing scheme to raise the reliability of a communication system and to improve transmission efficiency. The precoding scheme can be used irrespective of a spatial multiplexing rate in a multiple antenna system and increases a signal-to-noise ratio (SNR) of a channel. Examples of the precoding scheme include a codebook-based precoding scheme, which is used when feedback information is limited in a closed-loop system, and a scheme in which channel information is quantized and fed back. In the codebook-based precoding, the receiving end feeds an index of a precoding matrix, which is already known to both the receiving end and the transmitting end, back to the transmitting end to achieve an SNR gain.
In the 4th generation wireless communication systems, multi-cell interference is a big problem to be solved. PMI restriction or recommendation for the neighbor cells is a good scheme to handle the problem.
In this case, an efficient feedback scheme of PMI sets is required especially for the scenario in which the set of PMIs of the legacy mobile users with 4Tx antennas is restricted or recommended together with the PMIs of the mobile users with 8Tx antennas.